The gas chromatograph is perhaps the most useful analytical tool available today to the chemical process engineer. The gas chromatograph takes a fixed volume of sample gas, or liquid which can be vaporized, and introduces a fixed volume of sample gas into a separating column which contains a stationary phase of adsorbent material. The sample is transported through the separating column using a mobile phase carrier, and the individual molecules of the sample gas are adsorbed and then released at different times from the adsorbent stationary phase material in the column.
When the adsorbent material in the separating column and the operating parameters are properly selected, the separated components of the sample elute or emerge from the column completely separated in time from each other and from any other components that may be present. This eluting stream is passed through a detector and the relative response of the detector is sensed by an electronic unit and converted to a digital signal. The digital signal is a specific number that represents the concentration of a component in the sample which is true for one specific sample analysis. This number is generally stored in a memory register until updated.
If the sample update rate is high enough for a varying composition to change only slightly between each sample, the digital signal can quite accurately represent the composition vs. time behavior of the sampled quantity. The maximum sampling rate, however, is limited by the response speed of the sampling system and time required to analyze the sample. For a distillation column there can be a significant delay due to transportation of the sample to the analyzer plus the analysis time. For example, it may take two minutes for the sample to arrive at the analyzer another four minutes for obtaining the analysis, which results in a total delay of six minutes. Total delay times for distillation columns typically range from about three minutes to about thirty minutes.
To reconvert the digital signal obtained from periodic analysis of consecutive samples to a continuous or analog form for use in automatic process control, a zero order for a sample and hold circuit is usually sufficient. The zero order hold merely retains the value of the digital signal until the next sample changes it, therefor creating a dead time between analysis. Sample and hold techniques, however, result in poor control for fast responding distillation columns such as depropanizers, so that if composition control is desired on these fast changing columns, an analyzer controller must purposely be detuned for the column to remain stable.
Accordingly, it is an object of this invention to provide an accurate and essentially continuous composition analysis signal from a chromatograph analyzer which is suitable for use in controlling a fast responding distillation column.
It is further object of this invention to provide a dynamic inferential correction for the latest analysis provided by a distillation analyzer.
It is a still further object of this invention to infer concentration of a key component of a distillate product from a temperature measurement.
It is a still further object of this invention to determine the best point to locate a temperature sensor in a distillation column for inferring composition of a distillate product from a temperature measurement.